In coils for electrical or electronic equipment including inverter-related equipment such as high-speed switching devices, inverter motors, transformers, and the like, there are used insulated electric wires (insulated wires) composed of a so-called enamel wire, insulated wires having multi-covering layers including a layer composed of an enamel resin and a covering layer composed of another kind of thermoplastic resin other than the enamel resin, and the like as a magnet wire.
On the other hand, in recent years, advance of the electrical equipment represented by motors or transformers, has been progressed resulting in size reduction and improved performance. Thus, it becomes usual in many cases that insulated wires are used in such a way that the insulated wires are processed by winding (coil processing) and they are pushed into a quite small space to pack. Specifically, for an enhancement of the performance of a rotating machine such as a motor, it is required that a larger number of wound wires are accommodated into the slots of a stator. That is, improvement of the ratio of the sectional area of conductors to the sectional area of the stator slot (space factor) has been required to be particularly highly increased.
It has been lately attempted to use a rectangular wire in which the conductor has a shape similar to a rectangular (square or rectangle), as a means for increasing the space factor.
Use of a rectangular wire exhibits a dramatic effect in increasing the space factor. However, a corner portion of cross-section rectangular is extremely brittle to bending processing such as coil processing. Due to this, there is a problem that a coated film cracks when the rectangular wire is processed by applying strong pressure. In particular, it is known that the crack of the coated film tends to occur more easily as the curvature radius of the corner portion is smaller.
Furthermore, when the distance between conductors cannot be secured by making the thickness of the insulating coated film of the wound wire smaller, and the insulation performance cannot be secured. Furthermore, when the insulating coated film of the wound wire is damaged, discharge occurs from the exposed conductor of the wound wire.
In order to improve these electrical characteristics and mechanical characteristics, various investigations have been conducted.
For example, from a relation involving the relative permittivities of the enamel insulating layer and an extruded insulating layer and the temperature dependency of the tensile strength of the extruded insulating layer, it has been suggested that the thickness of the insulating coated film is made thick without impairing the insulation performance at high temperature, thereby the partial discharge inception voltage is increased, and the thermal-aging characteristics are improved (see Patent Literature 1).